In general, vehicles such as motorcycles and automobiles have tail lamps. In a case where two vehicles are running in the darkness, the preceding vehicle can emit beams from the tail lamps such that the driver of the following vehicle can recognize the existence of the preceding vehicle. However, in conditions of poor visibility due to rain, fog, or the like, even if the preceding vehicle emits the beams from the tail lamps, the driver of the following vehicle is likely to miss those beams, and to recognize the existence of the front-vehicle flag too late.
With respect to this problem, there is a technology for emitting beams from a lighting device of a vehicle onto an area of the road behind or at the side of the vehicle. According to this technology, the preceding vehicle emits the beams from the lighting device onto the area of the road behind or at the side of the preceding vehicle such that the emitted beams are reflected from the road, and if the driver of the following vehicle see those reflected beams, the driver can more clearly recognize the existence of the preceding vehicle, as compared to the case of seeing beams emitted from tail lamps. Hereinafter, beams which are emitted onto an area of the road around a vehicle will be referred to as “road irradiation beams”.
Patent Document 1 discloses a device usable as a device for emitting the road irradiation beams.
Patent Document 1: Japanese Patent Application Publication No. S63-213203A
However, in a case where a preceding vehicle always emits a constant road irradiation beam, sometimes it may be difficult for the driver of the following vehicle to recognize the existence of the preceding vehicle on the basis of the road irradiation beams, depending on the distance between the preceding vehicle and the following vehicle.
For example, in a case where the following vehicle comes very close to the preceding vehicle, even if beams emitted onto an area of the road behind the preceding vehicle are reflected from the road, the reflected beams are significantly deviated downward from the center of the field of view of the driver of the following vehicle. As a result, it may become difficult for the driver of the following vehicle to recognize the road irradiation beams and recognize the existence of the preceding vehicle on the basis of the road irradiation beams.
Meanwhile, in a case where the road irradiation beams are emitted onto positions on an area the road right behind the preceding vehicle, if the following vehicle is positioned far from the preceding vehicle, the reflected beams of the road irradiation beams from the road to enter the eyes of the driver of the following vehicle weaken. As a result, it may be difficult for the driver of the following vehicle to recognize the existence of the preceding vehicle on the basis of the road irradiation beams.